put water pressure gage or light on merc 650?

[Yachtzee]

I have a lat 60's merc 650 that runs very well. On the top of the block there is a plug that opens into the water jacket.

Can use this in anyway to add a water pressure idiot light or a gauge?
If so do I need to block the pee hole if I do this?
I have to walk back to the transom to see if water is comming out the pee hole, I'd like to keep an eye on it from the helm seat.